The sense of orientation

Abstract

This thesis is concerned with the sense of orientation. Vestibular, podokinetic and visual systems can all contribute to our sense of orientation in space. However, each provides the brain with different qualitative information in different reference frames. The aim of this work was to determine how these different sensory systems are fused to create a coherent internal representation of self-motion relative to the external environment and how this is used to guide locomotion. Vestibular stimulation with galvanic currents (GVS), podokinetic stimulation on a rotating platform, visual stimulation in a rotating visual scene all can evoke signals of rotation about the subject’s vertical axis. Healthy adult subjects were presented with a rotating stimulus of one or more afferent channels and perceptual, postural or locomotor responses during and following stimulation were recorded. GVS allowed systematic study of vestibular signals in isolation. Chapter 2 demonstrated these signals are interpreted within the CNS as head acceleration and revealed the peripheral and central vestibular adaption processes. Chapter 3 investigated the podokinetic after-rotation trajectory following a range of conditioning parameters and developed a model of podokinetic adaption incorporating vestibular feedback. Chapter 4 measured blinded locomotor responses during GVS which showed evidence of a podokinetic adaptation which summated with the vestibular response. Chapter 5 showed that a rotating vestibular signal in the presence of a stationary visual signal is rapidly recalibrated to the visual signal. Blinded after-rotation in the direction of the prior vestibular conditioning stimulus suggests the signal also undergoes adaptation according to vestibular processes. Rotating optic flow in Chapter 6 evoked illusions of self-motion but the dominance of vision on the sense of orientation diminished as the speed of optic flow increased. After-rotation trajectories revealed that the perception of self-motion which can be influenced by optic flow or the belief framework is the reference to which currently active sensory channels are calibrated. Chapter 7 investigates the locomotor responses following multisensory stimulation in conflicting and corresponding directions and demonstrated interactive influences of multiple inputs on the perception of self-motion. A conceptual model which incorporates the recalibration, adaptation and reweighting processes uncovered in the experimental chapters is presented in Chapter 8.